Rim & Disc brakes 1,049 words, estimated reading time: 7 min.
Before you parallel park a gargantuan old sedan without power steering, you crank the steering wheel as far as it’ll go, and it’s hard. Where do you grab it —at the center? in the middle?
No, you grab it outside, for max leverage. That’s sort of the idea behind rim brakes on a bike wheel. There’s more leverage at the rim, and rim brakes take advantage of it. Whether you’re turning a wheel or trying to stop it, you’re most effective far from the hub.
There are drawbacks. Applying friction to the metal rim heats it up, and if it gets too hot, the heated rim can pop the inner tube. If you’re descending the east side of Hwy 50 with 50lbs of gear, you need to continually brake so the bike doesn’t get up to 50mph and become a runaway. The continual braking heats up the rim, and the inner tube might pop, and the next thing you know, maybe the last thing you know, you’re topsy-turvy at 25mph. This is the image used to sell disc brakes, and as you can imagine, it’s effective.
(In the old days, experts told you to “pump your brakes,” the idea being to let your rims cool during the two-second or so off cycle. This gem of cycling lore never worked on severe descents: During the off cycle your speed increases way faster than the rim cools, and the braking/heating cycle ratchets up.)
Another drawback to rim brakes is that if the rim gets wobbly, braking gets jerky. It doesn’t happen out of the blue, and the spirit of turning lemons into lemonade, this can also be a reminder to fix your wheel.
With disc brakes, the rotors (the discs, the braking surfaces) keep the heat off the rim. They have much more braking surface than the rim does, and cooling vents, so they don’t get too hot—and even if they got orange-hot, who cares?— they can’t pop the inner tube.
Also unlike rims, the rotors don’t collect mud and water, so they’re unaffected by wet muck. Finally, since rotors-not-rims are the braking surface, wobbly rims don’t affect braking. These advantages seem to make a good case for disc brakes, and since they’ve proven themselves on heavy & fast cars and motorcycles, why not on bicycles?
That’s the quintillion-dollar question, and here’s a hundred-dollar answer–
If you tried to stop a rotating wheel by grabbing it near the center, you’d feel the power of the outer wheel’s leverage as it mangled your fingers and twisted your hand off at the wrist.
On a bike with disc brakes, the wheels and frame suffer the stress. All else equal, the closer to the hub the braking surface is and the smaller the wheel, the harder it is to stop. Disc brakes compensate with larger brake pads and braking surfaces, which concentrate forces at the hub.
And on the front wheel, the stresses imposed by center-of-the-wheel braking push the wheel downward toward the opening of the dropout. (In the early days of disc wheels there was a problem with wheels forcing their way past even the retaining bumps in the dropouts, leading to accidents. This has been solved with “through-axles.”)
Disc brakes in action also focus stress onto seat stays and fork blades, which can then buckle. Frame manufacturers address this with more mass, which brings up kind of a namby-pamby philosophical question: Which is better— a mechanical system that localizes stress on a small area, then bullies it into submission with bulk and beef, or one that disperses stress and spreads it out?
Most people buy overkill in all consumer products, because they don’t want to use equipment to its limit and then not be able to venture forth to the great beyond. But for 99 percent of the possible uses for 99 percent of bike riders, rim brakes on a bicycle aren’t near the limit. They’re, like…mid-field.
Rimmy brakes are a totally appropriate (and proven!) technology for riders who weigh 300lbs or less and move at bicycle speeds. Normal braking on a bike is on-again, off-again. The rim heats up, but it doesn’t get hot enough long enough to pop an inner tube. On the odd loaded descent of a Sierra pass, stop for a few minutes to enjoy the view while the rims cool.
Wheels may develop wobbles from crashes, blunt trauma, and loose spokes, but unless the wobble is severe, you tolerate the slightly less smooth braking for a mile or a month or even two, until you get around to fixing it. A bicycle has a large function cushion, and with the modern exception of carbon fiber parts, it’s not just Pass/Fail. Disc brakes are fine, but if the bike could speak for itself, it might request a rim brake. Since there’s so much variety out there, you don’t have to lock in, but you can’t just look at all bicycle-technical-opinion matters through an all-accepting lens, either. The fact is, rim brakes are getting pounded these days, but it’s a kind of artificial pounding by fashion and commerce.
Ultimately, you can expect the bicycle of the immediate future to become more of a high tech black box, with cables being replaced by hydraulics, and the visible levers and pulleys and other simple machines that combine into bicycle magic being hidden or replaced by electronics. The bicycle of the future will, absolutely, be shrouded in mystery and sold on reputation and faith, like a Samsung flat-screen TV. Until then, don’t dis the rim brake. It’s beautiful and it works, and today’s rim brakes are better than ever. They have to be.
——–BONUS SEMI-RELATED THOUGHT———– !
There’s a tendency to trust mechanisms you can’t see more than those you can, because when you see how something works, you see also the potential for failure. That’s one reason computers aren’t see-thru, and modern motorcycles shroud everything behind aerodynamic plastic. If you’re mechanically adept you might be more attracted to something you can figure out and fix, but more people aren’t that than are.
Also, for a different reason that might be related, lots of people trust complicated machines more than simple ones. What they want is push-button results, regardless of the complexity behind it. Maybe they figure a dumb uncle could design a machine with wheels and levers, but it takes a Ben Franklin or Bill Gates to work with electricity or electronics, so it must be better. Maybe it’s something else. This is just a BLUG, not a stone tablet.
Last time I wasn’t sure how many people read long posts, so Roman rigged up an auto-email count thing. I was surprised–about a thousand of you responded, and … I don’t need to keep score every time, but I may bring it back again if I start to feel like nobody’s home.
Grant
